Peripheral .alpha..sub.2 -adrenoceptors are associated with a variety of important physiological effects. Vascular .alpha..sub.2 -adrenoceptors when stimulated mediate vasoconstriction which gives rise to hypertension. Pancreatic .alpha..sub.2 -adrenoceptors modulate release of insulin. Platelet .alpha..sub.2 -adrenoceptors when activated cause platelet aggregation. Other .alpha..sub.2 -adrenoceptors affect gastrointestinal motility and fat cell metabolism. Molecules which selectively antagonize these peripheral .alpha..sub.2 -adrenoceptors offer a novel approach to the treatment of pathological conditions such as hypertension, diabetes, obesity, and disorders involving platelet aggregation and gastrointestinal motility.
.alpha..sub.2 -Adrenoceptors are located also in the central nervous system and mediate a variety of other physiological effects such as respiratory stimulation, psychomotor activity, increase in wakefulness and reduction in appetite.
Various compounds have been reported which affect .alpha..sub.2 -adrenoceptor activity. Thus, for example, certain benzoquinolizines have been reported in UK patent application Nos. 1435573, 2106909 and 2136804 to possess .alpha..sub.2 -adrenoceptor antagonistic activity. Generally, however, the .alpha..sub.2 -adrenoceptor antagonistic activity shown by most of the known compounds are non-selective, i.e., the compounds have a mediating effect on both central and peripheral .alpha.-antagonistic activity.